This invention relates generally to a method and apparatus for activating a liquid crystal display and, in particular, to a method and apparatus for making essentially uniform its crosstalk noise throughout the entire liquid crystal display to provide a display having uniform contrast and brightness regardless of the pattern of the display.
A conventional liquid crystal display matrix 200 is shown schematically in FIG. 20(a) and a conventional method for activating the display is shown in the timing diagrams of FIGS. 20(b)-20(g). Liquid crystal display 200 is formed with signal electrodes X1, X2 and X3 and scanning electrodes Y1, Y2 and Y3 orthogonal to the signal electrodes. Liquid crystal pixels are present at intersections of scanning electrodes and signal electrodes. A pixel at the intersection of scanning electrode Y2 and signal electrode X3 will be referred to a pixel Y2X3 for convenience. Cross hatched intersections of scanning electrodes and signal electrodes represent unselected pixels and unhatched intersections of scanning electrodes and signal electrodes represent selected pixels.
The scanning voltage, non-selecting voltage and selecting voltage are denoted VY, VX and -VX respectively. The waveforms of voltages applied to signal electrodes X1, X2 and X3 are represented by VX1, VX2 and VX3 respectively in FIGS. 20(g), 20(f) and 20(e) respectively. The waveforms of voltages applied to scanning electrodes Y1, Y2 and Y3 are represented by VY1, VY2 and VY3 respectively in FIGS. 20(b), 20(c) and 20(d) respectively.
A selecting period is the duration for which the selecting voltage is applied to a scanning electrode. During a first selection period, signal electrode Y1 is selected and scanning voltage VY is applied to electrode Y1. N voltage is applied to electrodes Y2 and Y3. To select a pixel at the intersection electrodes Y1 and X1, a selecting voltage -VX is applied to electrode X1 during the first selecting period. The pixels at the intersection of electrodes X2 and X3 with scanning electrode Y1 are to be unselected. Consequently, a non-selecting voltage VX is applied to these electrodes during the first selecting period.
The effective voltage applied to each pixel is equal to the difference between the voltage applied to the corresponding scanning electrode and the voltage applied to the corresponding signal electrode. Accordingly, a voltage of VY+VX (VY-(-VX)) is applied to the pixel at the intersection of signal electrode X1 and scanning electrode Y1 during the first selecting period. A voltage of VY+VX is of sufficient magnitude to activate a liquid crystal pixel. During the same selection period, voltage is not applied to scanning electrodes Y2 and Y3 and therefore pixels intersecting these electrodes will have a voltage of VX or -VX. The voltages are selected so that VY-VX is of insufficient magnitude to activate the liquid crystal pixel.
During the next selecting period, pixels Y2X2 are selected. Scanning electrode Y2 receives a selecting voltage VY and scanning electrodes Y1 and Y3 do not. A non-selecting voltage VX is applied to electrode Xl and a selecting voltage -VX is applied to electrodes X2 and X3. The applied voltage at the intersection of electrodes X2 and X3 with electrode Y2, will be VX+VY and pixels Y2X2 and Y2X3 at those intersections will be selected. Because a voltage of VY-VX is insufficient to activate the liquid crystal cells at intersections of scanning electrodes and signal electrodes, but a voltage of VX +VY is sufficient, only liquid crystal cell pixels at selected positions will become visible.
The method of operation during the third selecting when scanning electrode Y3 is selected is the same as with scanning electrodes Y1 and Y2. Pixels Y3X1 and Y3X2 are to be selected and signal electrodes X1 and X2 are at -VX. Pixel Y3X3 is to be unselected and the voltage at the intersection of scanning electrode Y3 and signal electrode X3 will have a voltage VY-VX which is insufficient to activate the liquid crystal pixel at that intersection and pixel Y3X3 will be unselected.
When a liquid crystal display matrix having a large area which can include hundreds of signal electrodes and scanning electrodes is activated by this conventional technique, undesirable crosstalk occurs between the scanning and signal electrodes. Crosstalk is caused by capacitance between the scanning and signal electrodes as well as the resistance of the wiring. Crosstalk noise from several sources on a single electrode can cancel out or increase in magnitude depending on the particular pattern of the matrix to be displayed at a portion of the panel and can change the effective value of voltage applied to different portions of the liquid crystal cell which affects the display characteristics, such as contrast ratio and brightness of the display. The localized difference in the effects of crosstalk lead to localized contrast variations of the liquid crystal display and therefore deteriorate the quality of the display.
As noted above, when a pixel is selected, it receives scanning voltage VY and selecting voltage -VX. When the pixel is not selected, voltage VX is applied. Referring to FIGS. 21(a) and 21(b), when the signal voltage at signal electrode X1 changes from -VX to VX or from VX to -VX, noise 70 and 70' is produced respectively at a scanning electrode as a result of capacitive coupling between the scanning electrode and the signal electrode. This will adversely affect the value of voltage applied to pixels by the scanning electrode. The magnitude of the noise generated when a signal pixel switches between selected and unselected voltage is substantially the same throughout the display provided that the electrodes have uniform resistance and that the capacitance between the electrodes is uniform. Accordingly, if the pixels are all uniformly switching between selecting and non-selecting voltages, the generation of noise will be uniform throughout the display and the quality of the display will be uniform and acceptable.
While the signal voltage at electrode X1 produces a noise 70 at a scanning electrode intersecting signal electrode X1, a wave form of voltage applied to signal electrode X2 can induce a noise 71 shown in FIG. 21(d) at the same scanning electrode as shown in FIG. 21(c). Further, a waveform shown in FIG. 21(e), applied to signal electrode X3 can induce a noise 72, shown in FIG. 21(f), in the same scanning electrode. The noise generated from signal electrodes to a scanning electrode will be the sum of the noises produced by the signal electrodes intersecting that scanning electrode.
Depending on the pattern of liquid crystal pixels to be selected over a given time interval, the noise can have different effects at different localized portions of the display. For example, if noise 70 and noise 71 are generated in the same scanning electrode, they will cancel out as shown in FIG. 21(g). If noise 70 and noise 72 are generated in the same scanning electrode, they will superimpose on each other to produce a noise of increased magnitude. Accordingly, because noise is generated differently at different portions of the same display, crosstalk will lead to localized contrast variations and an unsuitable display.
Conventional liquid crystal display activating methods therefore have inadequacies due to these shortcomings. Accordingly, it is desirable to provide an improved method of activating a liquid crystal display which avoids the shortcomings of the prior art and provides clear uniform displays that lack localized contrast variations caused by crosstalk.